1. Field of the Invention
The present invention relates to a process for the production of tin-plated wires, in particular wires for electrical applications which are constituted of non-ferrous metals, for example, such as copper, having a diameter of 0.1 to 1.5 mm, and preferably of up to 1.0 mm, through the intermediary of a two-step tin-plating.
2. Discussion of the Prior Art
From the disclosure of German Patent No. 29 31 939 there is already currently known the utilization of a two-step tin-plating process, in which a copper wire is drawn down to a nominal size and soft-annealed prior to the first tin-plating step, thereafter galvanically tin-plated in two steps, with a drawing sequence being introduced therebetween, subsequently drawn at high speed to its final size and annealed for a short time in an annealing stage, and finally is intensively cooled. Hereby, the annealing is effected for such a short time that although the copper wire has been fully annealed, nevertheless the tin coating has only been partly fused or smelted thereon. This will preclude the formation of a diffusion layer or junction between the copper and the tin.
In this prior art process it has been ascertained as being disadvantageous that the copper wire must be tin-plated twice relatively heavily, so as to necessitate a relatively high consumption of tin, while furthermore, because of the short-time fusing or smelting on, there is encountered the danger that there will be adversely affected the concentricity of the tin coating on the wire, and finally, because of the absence of a diffusion layer between the copper and the tin, there is encountered the danger of the disassociation of the tin-plated wire during the later soldering.
In the tin-plating of wires utilized for electrical applications it is common to employ two processes; in effect, the hot tin-plating or the galvanic tin-plating, of which each one possesses specific advantages and also disadvantages. Through the employment of hot tin-plating there is achieved the greatest possible adherence of the tin coating to the wire material because of the formation of an intermetallic diffusion layer. In this instance, there is obtained a good resistance against disassociation. Concurrently, however, this process possesses the disadvantage in that the application of the tin coating on the wire is not effected concentrically, but there are obtained locations with a thinner and locations with a heavier tinplating. During the artificial aging of such wires, such as are required by current German Industry Standards for certain classes of tin-plating, there can be produced disassociable surfaces in the region of these thin locations through the growth of the diffusion layer, which can lead to the rejection of the wires. In order to avoid the foregoing, it is necessary to impart a relative heavy coating of tin.
The galvanic tin-plating possesses the advantage in that the growth of the tin coating or layer on the wire is effected concentrically, and the coating evidences a good degree of freedom from porosity. However, the disadvantage of the galvanic process resides in the absence of the intermetallic diffusion layer, so that there can readily occur the peeling or chipping off of the tin coating during the bending of the wire, or a disassociation of the substrate or core material during test soldering.